HIV serological testing: Work on the development of an improved serological test has been successfully concluded. The competition ELISA test is superior in both sensitivity and specificity to licensed tests, including the Western blot. Viral pathogenesis: Work on the chimpanzee HIV model has suggested new directions for approaches to intervention. Findings are that infection appears to progress by discrete stages which may be variably immunoregulated and that cofactors or cellular immunity, or target cell selection may be fundamental. Also, in vitro tests of B- and T-cell immunosuppression by viral proteins and fragments produced by molecular biological techniques have been successful in the preliminary phase. Detailed characterization of mechanisms of immunosuppression are in progress. U.S. HTLV-I prevalence: A retrospective random sampling of the U.S. population (HANES-II) an a retrospective geographic drug- abuser population have been tested for HTLV-I antibody. Analysis in progress will indicate frequency of infection and its rate of change in these populations. HBLV prevalence: ELISA test have been successfully developed. Prevalence studies, geographically and epidemiologically oriented, are in progress.